Final answer:
Hib is rare in adults because they are likely to have developed immunity or have artificial passive immunity, whereas children have an immature immune system and insufficient antibody response, making them more susceptible. The Hib vaccine contributes to the rarity of this infection in children and adults.
Step-by-step explanation:
Haemophilus influenzae type B (Hib) is an encapsulated gram-negative bacterium that can be a primary cause of meningitis in children 2 months to 5 years old without the use of the Hib vaccine. Adults often have developed immunity to this bacterium. Infants, particularly those older than 2 months and up to 5 years of age, do not produce a sufficient protective antibody response due to their immature immune systems, which makes them susceptible to serious diseases such as meningitis caused by Hib.
The vaccination against Hib has made this infection rare in children in most countries. Adults produce a more effective immune response against Hib; their immune systems are more mature, which can include the encapsulation of the bacteria to stimulate a response.
By the age of five, most children have encountered Hib or similar pathogens enough times to develop immunity. Adults may also acquire immunity through artificially acquired passive immunity, where antibodies or activated T cells are injected to provide immediate protection against infection. Notably, Hib has a polysaccharide capsule that helps it avoid the immune system's defenses, making it more challenging for young children, with their developing immune systems, to effectively fight off the infection.